Terminology

• Joist: repetitive horizontal framing member
• I-joist: engineered joist with LVL or lumber flanges and an OSB or plywood web
• Floor truss: open-web truss of lumber chords and webs or metal webs; carries ducts and pipes through the depth
• Rim board: perimeter member at joist ends; LVL, LSL, or OSB rim
• Squash blocks / web stiffeners: blocking and stiffeners at concentrated loads and bearings
• Subfloor: structural panel sheathing over joists
• Glue-nailed / glue-screwed: subfloor bonded with construction adhesive plus fasteners to reduce squeaks and stiffen the floor
• Strongback: member fastened across trusses or joists to tie them and reduce bounce
• Live-load deflection limit: span divided by a number (L/360, L/480); higher divisors mean stiffer floors
• Gypcrete / topping: gypsum or lightweight concrete layer for sound, fire, and radiant heat

Standards

Ratings

• Deflection: L/360 is the common live-load minimum; L/480 is a common industry target for better performance
• Vibration: deflection alone does not predict how a floor feels; long spans need deeper joists, closer spacing, or vibration checks
• Subfloor: 23/32 in. tongue-and-groove rated sheathing common for joists at 16 in. to 24 in. o.c.
• Fire and sound: rated floor-ceiling assemblies combine joists, insulation, resilient channel, gypsum ceilings, and toppings per listed designs

Test standards

• AWC NDS: sawn lumber design
• ICC-ES or APA product reports: I-joist and floor truss capacities per manufacturer
• DOC PS 1 and PS 2: plywood and OSB subfloor
• ASTM E119: fire tests of floor-ceiling assemblies
• ASTM E90 / E492: airborne and impact sound

Materials

• Sawn lumber joists: short spans, simple framing
• I-joists: long, straight spans; light; consistent depth
• Open-web floor trusses: long spans with service chases
• OSB or plywood subfloor: structural sheathing
• Toppings: gypsum underlayment or lightweight concrete for sound and fire
• Hangers and connectors: joist hangers, straps, and hold-downs

Detailing

• Choose joist depth and spacing for stiffness, not only strength; check the manufacturer's L/480 span tables
• Glue and screw tongue-and-groove subfloor; leave panel gaps per the manufacturer
• Align bearing walls and beams below; avoid point loads on unsupported spans
• Plan MEP routing early; I-joist web holes and truss chases have limits
• Use direct-attached or resilient ceilings and toppings for sound ratings in multifamily
• Protect from weather; wet subfloors swell and squeak

Uses

Specific uses

• Single-family and townhouse floors
• Multifamily floors above podiums
• Light commercial and mixed-use buildings
• Renovations and additions
• Floors with radiant heat in gypsum toppings

Pros and cons

Pros

• Low cost and light weight
• Fast to install with common trades
• Engineered joists give long, straight spans
• Easy to route services in trusses

Cons

• Bounce and vibration on long spans
• Combustible; fire-rated ceilings needed in many buildings
• Poor impact sound isolation without toppings and ceilings
• Shrinkage and moisture movement cause squeaks and gaps

Manufacturing and cost

Construction

• Joists set on sill plates, beams, or hangers; blocking, rim board, and bridging installed
• Subfloor glued and fastened; toppings placed after rough-in

Main manufacturers and suppliers

• Weyerhaeuser (TJI joists)
• Boise Cascade (BCI and AJS joists)
• LP Building Solutions
• Huber Engineered Woods (AdvanTech subfloor)
• Simpson Strong-Tie (hangers and connectors)

Trade associations: American Wood Council and APA, The Engineered Wood Association.

Sources

• RedBuilt: understanding floor performance in wood construction, https://www.redbuilt.com/understanding-floor-performance-in-wood-based-construction-redbuilt
• Weyerhaeuser: floor performance technical bulletin SVC-4006, https://www.weyerhaeuser.com/application/files/9815/6417/8007/SVC-4006.pdf
• Boise Cascade: AJS and BCI joist span tables, https://www.kuikenbrothers.com/wp-content/stories/products/boise-cascade/ALLJOIST_2025_Residential_FloorSpanTables.pdf
• JLC: controlling floor vibration, https://www.jlconline.com/how-to/interiors/controlling-floor-vibration_o/
• Green Building Advisor: reducing floor vibrations, https://www.greenbuildingadvisor.com/question/reducing-annoying-floor-vibrations-interesting-paper
• Wikimedia Commons: Light Framing Construction Picture.jpg, by Nerv3d5053 (CC BY-SA 4.0), https://commons.wikimedia.org/wiki/File:Light_Framing_Construction_Picture.jpg